This invention generally pertains to the art of industrial inspection. More specifically, the present invention relates to a method for determining the surface quality of an object in a non-contacting fashion and an apparatus for practicing the method.
The invention is particularly applicable to a method for very precisely measuring the pitch and surface finish of a helicoidal surface, such as a blade on a marine propeller. However, it will be appreciated that the method and apparatus disclosed herein can also be used to measure the relative accuracy of any curved or flat surface wherein flatness, concentricity, parallelism, or uniformity are important. Thus, the method and apparatus disclosed herein can be used for the measurement of pitch and surface quality on aircraft propellers, axial flow pump impellers, process agitators for chemical production plants or refineries, sewage irrigation pump impellers, gas and steam turbine blades, torque converter blades, and runners, rotors, and stators in hydroelectric power turbines. Additionally, the invention disclosed herein can also be used in measuring the surface quality of cutting surfaces such as dies used in press forming operations as well as the surface quality of complex curved shapes such as body panels on vehicles or airplane wings and control surfaces. The invention can also be used to measure the relative smoothness of flat surfaces such as warehouse floors and the like.
The conventional propeller consists of two or more equally spaced radial blades which are rotated at a substantially uniform angular velocity. A marine propeller may be regarded as part of a helicoidal surface which, as it rotates, appears to "screw" its way through the water, driving water aft and the vehicle forward. The surface of the propeller blade facing aft experiences an increase in pressure thereby producing thrust. This surface is called the "face" of the blade. The forward-facing surface of the propeller is called the "back." The propeller face is commonly constructed as a true helical surface of constant pitch. The back is generally not a helical surface except in situations such as in submarine propellers where quietness of operation is a prerequisite. The shapes of blade outlines and sections of a propeller vary greatly according to the type of ship for which the propeller is intended.
The objective of propeller design is to produce a propeller that will convert the ship's power into thrust at the best possible efficiency. The propeller has to be strong enough to withstand all of the forces involved without causing blade fracture and needs to be shaped in such a way that the harmful effects of cavitation are avoided.
The passage of water past a propeller sets up a pressure reduction on the back of the blades and a pressure increase on the face of the blades. The largest contribution to the propeller thrust comes from the pressure reduction. If the pressure at any point falls to the pressure at which water vaporizes, then cavities of vapor are created in the water. This phenomenon is termed "cavitation". Such cavitation can be harmful to the efficient operation of the propeller because the subsequent collapse of the cavities can cause erosion of blade surfaces and increased vibration. Additionally, cavitation causes noise which is extremely detrimental in propellers intended for submarine applications.
Presently, the measurement of pitch and surface quality on marine propeller blades is done by a very complex machine and numerous man-hours are required to complete the measurement. It is understood by the inventor that the state of the art for measuring submarine propeller blades now requires the use of a contour scanning machine which costs on the order of $15 million and has a plurality of stylus arms or probes to measure the pitch and surface quality at a plurality of discrete locations on a propeller blade. It is also understood by the inventor that the state of the art process in submarine propeller inspection is a very time consuming and manpower intensive process. Additionally, even the most sophisticated machines are incapable of detecting minute imperfections in the surface quality of a propeller blade and only the fingertips of a highly trained human being are capable of detecting such flaws.
Generally, flaws, such as scratches, dents, pits, and cracks, can occur during the production of a propeller blade or they can result from damage to the blade surface subsequent to production. Any such flaws need to be detected and repaired before the propeller is put into operation initially or is returned to operation, as the case may be.
Accordingly, it has been considered desirable to develop a new and improved method and apparatus for determining the surface quality and, if desired, the pitch of an object which would overcome the foregoing difficulties and others while providing better and more advantageous overall results.